app: [Wayland] account for fallback decoration height in window sizes

Pass through a fallback window decoration height to the Wayland backend,
so that it can account for it when determining surface size.

Fixes: https://todo.sr.ht/~eliasnaur/gio/435
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/os.go

@@ -43,6 +43,9 @@ type Config struct {
 	CustomRenderer bool
 	// Decorated reports whether window decorations are provided automatically.
 	Decorated bool
+	// decoHeight is the height of the fallback decoration for platforms such
+	// as Wayland that may need fallback client-side decorations.
+	decoHeight unit.Dp
 }
 
 // ConfigEvent is sent whenever the configuration of a Window changes.